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that we still use to describe embryos and raised scientific questions that we are still
trying to answer today. In Dissertatio Inauguralis, published in January, Pander
coined the term blastoderm (blastoderma) to define the membrane that “... the
Embryo chooses as its seat and its domicile, contributing much to its configuration
out of its own substance” (p. 21) (Pander 1817b). Subsequently, he showed that the
blastoderm is divided into three “lamellae,” called germ layers (Keimhäute) (Pander
1817a). Although he was not the first to observe embryonic membranes, Pander was
the first to understand their significance:
At the twelfth hour the blastoderm consists of two entirely separate layers, an inner one,
thicker, granular and opaque, and an outer one, thinner, smooth and transparent. The latter,
because of its development and for the sake of greater accuracy of description, we may call
the serous layer and the former the mucous layer ... There arises between the two layers of
the blastoderm and third middle one in which the blood vessels are formed, which we there-
fore call the vessel-layer; from its origin events of the greatest importance subsequently occur
... Actually there begins in each of these three layers a particular metamorphosis, and each
one strives to achieve its goal; only each is not yet sufficiently independent by itself to produce
that for which it is destined. Each one still needs the help of its companions; and therefore all
three, until each has reached a specific level, work mutually together although destined for
different ends (pp. 5–6) (Translated by Jane Oppenheimer (Oppenheimer 1940 )).Not only did Pander describe the three germ layers, but he realized that they
originated from the blastoderm: “For before incubation this membrane consists of a
single layer, made up of granules which cohere to each other by their own viscosity.
As incubation progresses, however, there originates from this another layer, more
delicate but firmer in structure, so that at a specific time the blastoderm can be
divided by a fairly long maceration into two layers.” (Pander 1817b; Oppenheimer
1940 ). Finally, he showed that each layer has distinct contributions to the embryo:
the mucous layer, which we now call the endoderm, generates the gastrointestinal
system; the vessel layer, or mesoderm, produces the blood and blood vessels and the
serous layer, or ectoderm, gives rise to the epidermis and the central nervous sys-
tem. We now understand the fates of the three germ layers in much greater detail
(Fig. 7.1), but Pander’s overall outline has withstood the test of time. Pander also
recognized that mutual interactions between these three tissues were likely to be
important for normal development, since they “work mutually together” to achieve
their “different ends.”
Pander’s work was the culmination of an exhaustive series of observations on
over 2000 live chicken embryos, conducted in collaboration with his doctoral advi-
sor, the anatomist Ignaz Döllinger (1770–1841), and a skilled artist, Eduard d’Alton
(1772–1840). They focused on the earliest stages of development and described
only what they could see at a particular time without any preconceived notions
about the fates of the tissues at later stages (Schmitt 2005 ). This unbiased approach
was the intellectual breakthrough that allowed them to understand the biological
significance of the embryonic membranes. Pander also used technical innovations
that permitted him to see the embryo in greater detail than previously possible. He
used a new dissection technique developed by Döllinger to isolate embryo and
observe it on a black background for improved contrast (Schmitt 2005 ). Each of the
three men viewed the embryos independently, and they consulted with each other to
W. Tseng et al.